I recall spending a fair amount of time playing on building sites. A half built street was one hell of an adventure playground; with floorless rooms to explore, piles of bricks and sand to climb on, jump off and over, dumper trucks and diggers play in/on. There was never any "don't play on that building site" from our parents. It was just as acceptable as playing on the swings or in the back field... when did they start fencing them off?